By MELINDA BECK
Doctors long have assumed that saturated fat and cholesterol in red meat are what raise the risk of heart disease. But a study out Monday in the journal Nature Medicine fingers another culprit: c
Carnitine, a compound abundant in red meat that also is sold as a dietary supplement and added to energy drinks.
Carnitine typically helps the body transport fatty acids into cells to be used as energy. But researchers at the Cleveland Clinic found that in both humans and mice, certain bacteria in the digestive track convert Carnitine to another metabolite, called TMAO, that promotes atherosclerosis, or a thickening of the arteries.
The researchers, led by Stanley Hazen, chief of cellular and molecular medicine at the Cleveland Clinic’s Lerner Research Institute, examined records of 2,595 patients undergoing cardiac evaluations. They found that the more TMAO in their blood, the more likely they were to develop cardiovascular disease, heart attack, stroke and death.
Many studies over the years have linked consumption of red and processed meat to cardiovascular disease and some cancers. The Harvard School of Public Health reported last year that among 83,000 nurses and 37,000 male health professionals followed since the 1980s, those who consumed the highest levels of red meat had the highest risk of death during study, and that one additional serving a day of red meat raised the risk of death by 13%.
The new findings don’t mean that red meat is more hazardous than previously thought, but they may help explain the underlying risk, which some researchers have long thought was greater than the saturated fat and cholesterol content could explain.
Dr. Hazen speculated that Carnitine could be compounding the danger. “Cholesterol is still needed to clog the arteries, but TMAO changes how cholesterol is metabolized—like the dimmer on a light switch,” he said. “It may explain why two people can have the same LDL level [a measurement of one type of cholesterol], but one develops cardiovascular disease and the other doesn’t.”
One surprising finding, Dr. Hazen said, was how long-term dietary patterns affected the amount of TMAO-producing bacteria in the gut and thus magnified the risk. In the study, when a longtime omnivore consumed an eight-ounce steak and a carnitine supplement, both his bacteria and TMAO levels rose considerably. But when a vegan voluntarily ate the same combination, he showed no increase in TMAO or bacterial change.
“Vegans basically lose their ability to digest” carnitine, said Dr. Hazen.
The study, sponsored by the National Institutes of Health, didn’t assess how little red meat people could consume and still have elevated TMAO. Nor did it look at how long someone had to abstain from red meat to end the process. “We know it will be longer than one week, but shorter than one year,” Dr. Hazen said.
Trade groups for meat producers have questioned the link between red meat and cardiovascular disease on the grounds that studies that ask people to recall what they ate over long periods are imprecise.
As a dietary supplement, Carnitine is designated as “generally regarded as safe” by the Food and Drug Administration, but few studies have looked at its long-term safety. A 2006 risk assessment found no adverse effects when subjects consumed 2,000 milligrams a day for six months.
Ads for supplements promote Carnitine as helping boost energy levels, particularly in endurance sports, and assisting in recovery after intense exercise; some also claim that it helps reduce belly fat, shed pounds and improve brain function.
Duffy MacKay, vice president for scientific and regulatory affairs at the Council for Responsible Nutrition, a trade group for the supplement and energy drink industry, called the Nature study “a new, emerging hypothesis,” but said the researchers were drawing large conclusions from small studies of mice, bacteria and human biomarkers.
“The concept that one component of your diet, or one molecular, is responsible for your health woes is questionable,” Mr. MacKay added.
Dr. Hazen noted that some energy drinks have more carnitine in a single can than a porterhouse steak. “I worry about what happens in 10, 20 or 30 years of consumption,” he said.
He said humans generally have plenty of carnitine in their diet, which also is found in small amounts in nuts, beans, vegetables and fruit, and don’t need to take it in supplement form.